In communications and computer related fields, a need exists for combining electrical and optical technologies. In particular, deregulation of telecommunication operations has resulted in widespread use of Internet, e-mail, data transmission, cable networks and wireless telephones. These developments have driven a strong demand for increased network capacity, speed, and bandwidth, and to meet this demand, telecommunications services have been installing optical networks because they anticipate greater capacity, and more cost effective features than traditional hard wired networks. Current research anticipates even wider use of high speed optoelectronics in which photons, rather than electrons will pass signals from board to board, or chip to chip thereby avoiding the delays of conventional wiring. Electrical signals from a processor will modulate a light or laser beam which would shine through air, a waveguide, or an optical fiber to a photodetector, which in turn will pass signals on to the electronics.
The resulting demand for optical and optoelectronic components, and the associated packaging technology to meet the unique needs of these applications far exceeds current capabilities. Broadband performance, high density interconnection, and precise alignment of the optical components present significant challenges to existing assembly technology. The cost of producing optical and optoelectronic modules is dominated by the cost of optical interconnections and packaging the devices, rather than the cost of the components. Ultimately, the cost of packaging and assembly of optoelectronic devices will need to be comparable to that for electronic components, and must rely on much of the technology and automation from the existing industry.
Currently, the manufacture of optoelectronics modules requires that an optical fiber be properly aligned to an optoelectronic chip, namely an integrated circuit. Optical signals received or transmitted over optical fibers are coupled to an optoelectronic chip where they are converted to electrical signals. Optical signal coupling is optimized by precise alignment with minimum attenuation.
A hybrid optoelectronic package is formed by interfacing an optical fiber with an optoelectronic device, but preferably direct contact is not made between the chip surface and fiber end in order to avoid damage to the chip surface.
A significant aspect of packaging optoelectronic devices or modules involves aligning an active circuit to an optical fiber or waveguide, and electrically and mechanically bonding the circuit to a substrate having interconnection circuitry. One bonding approach which has been used in electronic components, and which is gaining favor in optoelectronic devices is flip chip connection by solder bonding. Flip chip bonding allows for direct connection of the active surface of a semiconductor device to a substrate. The contact pads on each chip include solder bumps which are mated to pads on the substrate, and the solder is reflowed to ensure electrical and physical contact. Surface tension in the molten solder causes the opposing contact pads to be aligned with good precision.
Prior art assembly of optoelectronic devices included one or more optical fibers positioned on a substrate having wiring interconnections, and the circuit chip(s) bonded to the substrate through wire bonds or flip chip interconnection. Some early work on flip chip assembly of optoelectronic devices provided a silicon substrate having a groove anisotropic ally etched, the fiber aligned in the groove, a film deposited to cover and secure the fiber, and conventional solder mount of a flip chip to the substrate. Despite the advantage of similar thermal expansion between the substrate and chip, high cost, misalignment, and some reliability issues have precluded the success of this process.
It has long been recognized that in order to gain the desired high frequency response for optoelectronic devices, minimizing the interconnection distance provides the best approach, and that flip chip assembly, wherein the active circuit of the chip is aligned directly atop the end of the fiber, offers the best solution for these high speed devices.
However, the use of flip chip bonding for optoelectronic assembly has revealed limitations which were not of concern in electronics, namely bonding optical and optoelectronic devices involves even more demanding alignment tolerances than electronics. Alignment in the plane normal to the substrate (“z” direction) typically has not been of concern in electronics, but in optoelectronics alignment in each of the “x”, “y”, and “z” directions is critical to optical coupling efficiency.
One form of prior art optoelectronics packaging, illustrated in FIG. 1a included an aperture 101 formed through a substrate 100, and an optical fiber 120 secured within the opening. The substrate with fiber could be aligned in the “x” and “y” directions, but improper alignment in the “z” direction could allow the fiber to touch the active surface of an optoelectronic chip 110 and damage the device. Alternately, in FIG. 1b the end of the optical fiber 121 positioned through the aperture 101 was much further from the active surface of the chip 111, and could result in misalignment in the “z” direction, and signal loss.
More recently, attempts have been made to position optical fibers or waveguides in different types of substrates, and to flip chip mount the active components onto non-CTE (coefficient of thermal expansion) matching substrates. This requires use of an underfill polymer to control stress on the solder joints resulting from the mismatch. FIG. 2 illustrates the fiber 220 positioned in the substrate 200 and the optoelectronics chip 210 mounted using flip chip solder bumps 240. An underfill material 250 having a costly light transmitting filler surrounds the solder bumps. The assembly is not unlike that used for flip chip semiconductor devices, except that attenuation of light is of utmost importance in optoelectronic device, and therefore, only highly specialized filler for optical transmission may be formulated into the underfill polymer. While the specially formulated underfill material attempts to minimize attenuation of the light beam, it is not as effective as is required for high speed transmission devices.
Not only is the underfill material very expensive, but a slow curing process at a low temperature must be used in order to avoid misalignment of the filler particles and resulting diffraction of the light. The materials are costly, and the process is both time consuming and labor intensive, and thus is not amenable to high volume production.
It would be very beneficial to this fast growing industry if a reliable, cost effective technique for assembling a flip chip optoelectronic device were provided.